Electrophotographic marking is a well-known, commonly used method of copying or printing documents. Electrophotographic marking is performed by exposing a charged photoreceptor with a light image representation of a desired document. That light image discharges the photoreceptor, creating an electrostatic latent image of the desired document on the photoreceptor's surface. Toner particles are then deposited onto that latent image, forming a toner image. That toner image is subsequently transferred from the photoreceptor onto a substrate, such as a sheet of paper. The transferred toner image is then fused to the substrate, usually using heat and/or pressure, thereby creating a permanent image. The surface of the photoreceptor is then cleaned of residual developing material and recharged in preparation for the production of another image.
When fusing toner onto a substrate it is beneficial to heat the toner to a point where the toner coalesces and become tacky. The heat causes the toner to flow into the fibers or pores of the substrate. Adding pressure increases the toner flow. Then, as the toner cools it becomes permanently attached to the substrate. To produce the heat and pressure for fusing, most fusers include a heated element and a pressure-inducing element that act together to form a nip. When a toner bearing substrate passes through that nip, heat from the heated element and pressure within the nip fuses the toner with the substrate.
One type of fuser uses a heated roller, called a fuser roller, and a nip-forming roller called a backup or pressure roller. Fuser rollers have been heated in different ways, including the use of an internal radiant heater, inductive heating, and by an internal resistive heating element. While fusers having a fuser roller and a backup roller have been very successful, they generally suffer from at least one significant problem: excessive warm-up time. When a typical prior art fuser roller using machine is turned on it might take several minutes for the fuser roller to warm-up to a point at which fusing can be performed. Furthermore, to conserve energy and to prolong the life of various internal components it is beneficial to remove power from the fuser roller heater when the fuser roller is not being used. However, it could then take several more minutes to re-heat the fuser roller. These delays are highly objectionable.
One approach to reducing fuser warm-up times is to pass electrical current through a resistive heating layer on a fuser roller such that the nip is directly heated. While such an approach is beneficial, it is difficult to implement in a long life fuser roller. This is partially because long life fuser rollers usually have metallic cores made from structurally rugged materials such as steel, stainless steel, or aluminum. Such metallic core fuser rollers are thermally conductive, and thus conduct heat away from the nip, and electrically conductive, and thus tend to short out resistive heating layers. Therefore, an insulating layer over the metallic core is usually used to prevent electrical shorting and excessive heat loss. Furthermore, to prevent damage to the resistive heating layer and toner sticking to the fuser roller, the resistive heating layer is usually coated with a protective release layer. However, even then such fuser rollers require a significantly long warm up time. At least one reason for the significantly long warm-up time is the materials used in prior art rapid warm up metallic core fuser rollers.
Some of the most effective ways of improving the warm-up time of metallic core fuser rollers are to reduce the heated thermal mass and/or to increase the thermal insulation. These properties depend on the materials used to make the fuser roller. The choice of materials suitable for use in metallic core fuser rollers is constrained by the material's function (electrical and thermal insulation, toner release, conformance), operating conditions (high temperature and pressure), and longevity requirements. One high conformance, high temperature material with good insulating and release properties is volume graft. Volume graft is a volume grafted elastomer invented by S. Badesha et. al. and described in U.S. Pat. No. 5,744,200. Volume graft has the beneficial characteristics of being thermally and electrically insulating, highly conformal, and thermally stable. Therefore, a rapid-warm up fuser roller having a metallic core with a volume graft insulating layer would be beneficial.